Method and apparatus for producing sheet metal components

ABSTRACT

A method and apparatus for making sheet metal components includes heating a sheet of hardenable steel to its austenitizing temperature and hot forming the same in a hot forming tool to define a formed workpiece. The formed workpiece is held in the hot forming tool for a first holding time. The formed workpiece is then removed from the hot forming tool and immediately placed in a cooled, form holding tool and is held therein in a closed condition for a second holding time. The form holding tool may be constructed from a material that has greater heat conductivity than that of the hot forming tool.

CLAIM OF PRIORITY

Applicants hereby claim the priority benefits under the provisions of 35U.S.C. §119, basing said claim of priority on German Patent ApplicationSerial No. 10 2009 021 395.3, filed May 14, 2009. In accordance with theprovisions of 35 U.S.C. §119 and Rule 55(b), a certified copy of theabove-listed German patent application will be filed before grant of apatent.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus for producingsheet metal components.

Form-hardening and press-hardening of sheet metal components areestablished in the automobile industry. However, the associatedcomponent manufacturing costs are relatively high, so there are effortsto reduce cycle times for the production process. One reason for thecurrent long cycle times is that a certain holding time in the formingtool is required during form-hardening in order to extract or remove theheat from the formed workpiece that is to be hardened. Becausetransformable steels must be used for press-hardening, they must beaustenitized. That is, they must be heated to their material-specificaustenitizing temperature, which is the 950° C. range. During forming,the workpieces undergo rapid cooling during which the material structureleaves the austenitic range. Due to cooling to a temperature range ofbetween 100° C. and 200° C., the material transforms into a martensiticstructure that provides the desired high-strength components. Onematerial that is very well suited for this method is 22MnB5. Thecomponents produced in this manner are used especially in side impactprotection in “A” and “B” columns, rocker panels, frame parts, bumpersand bumper supports, door reinforcements, and transverse beams and roofsupports for motor vehicles.

Because hardening is desired, relatively large quantities of heat mustbe removed from the workpiece in the shortest possible period of time.Although the cooling curve drops sharply at first, it flattens later onduring the cooling cycle. This means that as the holding periodincreases, the amount of heat removed continues to decrease per unit oftime. This leads to relatively long holding times of 15 to 20 secondsduring which the forming tool is closed and not available for producingother sheet metal components. This is particularly true for transferpresses in which workpieces are transferred between workpiece linesusing transfer bars.

Theoretically, it is possible to remove the sheet metal components fromthe forming tool at elevated temperatures, and then let them cool inambient air. However, the components are still warm and will warp,leading to increased rejections because of defects.

SUMMARY OF THE INVENTION

One object of the present invention is a method and apparatus forproducing sheet metal components using shorter cycle times whileachieving the same shape accuracy.

This object is attained using a method having the steps or measures setforth in patent claim 1 herein.

The subject-matter of patent claim 4 herein is a suitable apparatus forperforming the subject method.

The subordinate claims relate to useful, non-obvious embodiments of theinventive concept or thought disclosed herein.

In the present inventive method, a workpiece made from steel sheet witha transformable structure is austenitized in a known manner. That is, itis heated to a temperature that is greater than its austenitizingtemperature, and is placed in a forming tool for forming andpress-hardening the workpiece. The workpiece is cooled very rapidlyinside the forming tool. To this end, the forming tool, which is a presshaving a top die and a bottom die, is intensively cooled to attain thedesired cooling speeds. The austenitic structure transforms into amartensitic structure. The material is thus formed and hardened in thesame tool. The workpiece must now be held in the forming tool until afirst holding time has elapsed. Then, the formed workpiece, which hasalready been press-hardened, but is still warm, is removed from theforming tool, and immediately placed in a cooled, form-holding tool. Theformed sheet metal component is held in the form-holding tool until asecond holding time has elapsed. It is preferable that this secondholding time is about the same as the first holding time. The workpiececools inside the form-holding tool to a final temperature, that permitsit to be removed without the finished workpiece warping in an undesiredmanner. The material of the form-holding tool preferably has greaterheat conductivity than the material of the forming tool.

This procedure has the advantage that the forming tool, which is in highdemand, does not need to be closed for 15 to 20 seconds. Rather, after arelatively brief holding period it can be re-opened, so that it isavailable again for forming another sheet metal component after a briefperiod of time. The cycle time for producing sheet metal components isthereby cut, and in the best case can be halved, by using two tools.These two tools are specifically a first tool that is configured forforming, and a second tool that is configured for cooling and, wherenecessary, sizing as well. This feature of the present invention isclarified by the following example.

In a transfer press, the workpieces are moved between two forming toolsusing two cooperating transfer bars that work in pairs. The transferoccurs between the strokes of the transfer press. Assume for instancethat the transfer time between two tool stages is 5 seconds, and thatthe holding times in the first and second forming tools are the same andare for instance 10 seconds. Given this assumption, the transfer time isa total of 15 seconds. These 15 seconds are for transferring the steelsheet to the first forming tool, from there to the second forming tool,and finally for removing it from the second forming tool. There are also2×10 seconds of holding time. This means that the first workpiece ispress-hardened after 15+20=35 seconds.

One component is finished every 15 seconds based on the cycle for thetransfer bars.

Compare this to a single-stage press tool that is also held closed for20 seconds, and in which there will also be a 5 second transfer time forboth loading and removal. In this case, the first workpiece ispress-hardened after 20+2×5=30 seconds. Each subsequent component willbe produced after an additional 25 seconds.

In contrast to the present inventive method, when using only one formingtool, the production process takes longer. In this example, it is 10seconds longer due to the longer holding time. Given these assumedparameters, it is possible to increase production speed by 40% relativeto a 25 second-cycle.

Another significant aspect of the present invention is that the toolscan be better matched to their primary purpose. In other words, theforming tool can be produced from a very strong and durable materialthat is specifically matched to the requirements of hot-forming andform-hardening.

In the first tool, it is important that the component be cooled asuniformly as possible. Therefore a plurality of cooling bores andcooling channels are provided in the first tool for this purpose.

In the second tool, it is no longer primarily an issue of cooling theworkpiece as uniformly as possible. Rather, the issue is bringing theworkpiece to the final temperature as rapidly as possible so that itdoes not warp. Therefore, in the second tool, it is possible to use adifferent material and a different cooling geometry than the first tool.The material of the second tool does not necessarily have to have thehigh strength and hardness like as the material for the first or formingtool. Instead, due to the lower mechanical stress, the second tool canbe made from a material that has greater heat conductivity. A coppermaterial may even be used, which has significantly higher heatconductivity than that of the first or forming tool.

During the development of the present invention, efforts were made toreduce the cycle times during press-hardening as much as possible.However, for quality reasons, it must be understood that thetransformation to a martensitic structure must be as complete aspossible, and that certain production tolerances are not exceeded.Different holding times for the holding tools are needed, depending onthe shape and material of the steel sheet to be formed. The inventionseeks to have the forming tools held closed for press-hardening for nomore than 20 seconds, preferably no more than 15 seconds, and especiallyno more than 10 seconds. The holding times in the forming tool and theholding times in the downstream form-holding tool are preferablysynchronized. For example, if in the past a holding time of 20 secondswas required during single-step production using only a single formingtool, the goal of the present invention is for this total holding timeto be halved. In addition, there is also the time for opening andclosing the tools, and handling the workpiece to transfer it from theforming tool to the form-holding tool. However, the total time stillyields a shorter cycle time with respect to the forming tool, so it ispossible to accelerate the overall production process.

In the present invention, the press-hardened workpiece is cooledprimarily in the form-holding tool. The form-holding tool prevents theworkpiece from warping. In the present invention, it is also possible tosize the press-hardened workpiece in the form-holding tool in order tobetter maintain the desired production tolerances.

The present inventive apparatus for performing the method isdistinguished in that the heat conductivity of the material for theform-holding tool is greater than that of the material for the formingtool. This relates primarily to the areas of the tools that come intocontact with the steel sheet. Naturally, it is also possible to producethe core of a tool from a different workpiece than the areas that comeinto contact with the workpiece that is to be shaped or held or sized.It is possible to select material combinations, especially in the areaof the form-holding tool, in which the surface heat conductivity, thatis, the heat conductivity in those areas that comes into contact withthe workpiece, is high, but decreases towards the core of the tool. Forinstance, a copper cover layer could be paired with a support frame, inthe form of a steel core, that is deeper in the tool.

Another distinct advantage of the present inventive apparatus is thatthere may be fewer cooling bores and/or grooves in the form-holding toolthan in the forming tool. Fundamentally, both the forming tool and theform-holding tool are liquid-cooled in order to be able to remove thegreatest quantity of heat in a short period of time. Because of the factthat the form-holding tool itself has no role in the forming of thesteel sheet, it is possible to choose larger cooling cross-sections andsimpler cooling geometries, such as shaft cooling for instance, whereina series of grooves are provided in the surface of the form-holdingtool. Cold water flows through the grooves. The same is true concerningthe arrangement of the cooling bores. It is possible to provide only afew central cooling bores, so that the machining of the form-holdingtool is significantly less complex than that of the first or formingtool.

These and other advantages of the invention will be further understoodand appreciated by those skilled in the art by reference to thefollowing written description, claims, and appended drawings.

The invention shall be explained in greater detail in the followingusing the exemplary embodiments depicted in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a portion of a forming tool embodying thepresent invention;

FIG. 1A is a schematic view of a portion of a form-holding toolembodying the present invention; and

FIG. 2 is a cooling curve graph.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For purposes of description herein, the terms “upper,” “lower,” “right,”“left,” “rear,” “front,” “vertical,” “horizontal,” and derivativesthereof shall relate to the invention as oriented in FIGS. 1 and 1A.However, it is to be understood that the invention may assume variousalternative orientations and step sequences, except where expresslyspecified to the contrary. It is also to be understood that the specificdevices and processes illustrated in the attached drawings, anddescribed in the following specification, are simply exemplaryembodiments of the inventive concepts defined in the appended claims.Hence, specific dimensions and other physical characteristics relatingto the embodiments disclosed herein are not to be considered aslimiting, unless the claims expressly state otherwise.

FIG. 1 provides a highly simplified and schematic depiction of thebottom die for a hot forming tool 1 with a workpiece 2 made of ahardenable steel sheet. In FIG. 1, workpiece 2 has already been formedin the depicted U shape inside the forming tool 1. The top die for theforming tool 1 is not shown. The workpiece 2 is hardened in the formingtool 1. To this end, the workpiece 2 has been austenitized in advance,that is, it has been heated to a temperature greater than itsaustenitization temperature. The forming process, and an accelerated orrapid cooling process both occur inside the forming tool 1. The quantityof heat that occurs is removed from the forming tool 1 via liquidcooling (not shown in greater detail).

The workpiece 2 that has been formed and press-hardened in this manneris then immediately placed in a form-holding tool 3, which is depictedon the right in FIG. 1A. The form-holding tool 3 has the same contour asthe form-giving forming tool 1. However, its purpose is not forming.Rather, the purpose of form-holding tool 3 is to fix and even, and whennecessary, to size the still-warm workpiece 2 in order to prevent theworkpiece 2 from warping due to cooling.

The top die is not shown in this depiction of the form-holding tool 3,either. It is obvious that the workpiece 2 is enclosed between top dieand bottom die, both in the forming tool 1 and in the form-holding tool3.

The progression of the production method can be explained using FIG. 2.

FIG. 2 provides a cooling curve for a workpiece 2. The workpiece 2 isinitially heated to a temperature that is at least equal to theaustenitizing temperature (Ac3). The austenitic structure of theworkpiece 2 is to be transformed to a martensitic structure. This occursbetween a martensite start temperature Ms and a martensite finishtemperature Mf. It can be seen that the cooling curve drops sharplyinitially, passing through the range for the martensite transformationrelatively quickly. However, then the cooling curve flattens. The actualpress-hardening has already concluded at a temperature slightly belowthe martensite finish temperature Mf. The press-hardened workpiece 2 isremoved from the forming tool 1 after the holding period, labeled “A,”and is immediately placed into the form-holding tool 3, without anyintermediate storage. The holding time in the form-holding tool 3 isidentified with “B” in the depiction in FIG. 2. It can be seen that theholding times “A” and “B” are essentially equal, so that the cycle timefor the forming tool 1 is essentially half that of a prior forming toolin which complete cooling occurs to the final temperature.

In the foregoing description, it will be readily appreciated by thoseskilled in the art that modifications may be made to the inventionwithout departing from the concepts disclosed herein. Such modificationsare to be considered as included in the following claims, unless theseclaims by their language expressly state otherwise.

LEGEND

-   -   1—Forming tool    -   2—Workpiece    -   3—Form-holding tool    -   A—Holding time in the forming tool    -   B—Holding time in the form-holding tool    -   Ac3—Austenitizing temperature    -   Ms—Martensite start temperature    -   Mf—Martensite finish temperature

1-6. (canceled)
 7. A method for making sheet metal components,comprising: heating a sheet of hardenable steel to a temperature greaterthan the austenitizing temperature of the steel; hot forming the heatedsteel sheet in a hot forming tool to define a formed workpiece; holdingthe formed workpiece in the hot forming tool for a first holding time;after said holding step, removing the formed workpiece from the hotforming tool, and immediately placing the formed workpiece into a cooledform holding tool; and holding the formed workpiece in the closed formholding tool for a second holding time.
 8. The method as set forth inclaim 1, wherein: said holding steps include making the first and secondholding times substantially equal.
 9. The method as set forth in claim1, wherein: said second named holding step includes sizing the formedworkpiece in the form holding tool.
 10. An apparatus for making sheetmetal components, comprising: a hot forming tool constructed from afirst material and configured for press hardening a sheet of hardenablesteel into a formed workpiece; a form holding tool constructed from asecond material and configured to closely receive therein the formed andhardened workpiece; and wherein the second material of the form holdingtool has greater heat conductivity than the first material of the hotforming tool.
 11. The apparatus as set forth in claim 10, wherein: thesecond material of the form holding tool is less strong and less hardthan the first material of the hot forming tool.
 12. The apparatus asset forth in claim 10, wherein: the form holding tool has fewer coolingapertures than the hot forming tool.